Silk screen printing machine

ABSTRACT

A silk screen printing machine which includes an inking blade having a concave trowel configuration for more efficiently applying ink to a silk screen stencil, and a pressure roller which cooperates with a paper-carrying drum to force ink through the stencil onto paper carried by the drum. A solvent feeding porous wiper, which continuously feeds a cleaning solvent by capillary action, bears against the periphery of the pressure roller to clean residual ink from this roller after it has pressed ink through the silk screen onto the paper.

United States Patent [7 2] Inventor Stancil 1. Ramsey 4009 Harvey Parkway, Oklahoma City, Okla. 73118 [2]] App]. No. 34,187 [22] Filed May 4, 1970 [45] Patented Oct. 5, 1971 Continuation-impart of application Ser. No. 697,232, Jan. 11, 1968, now abandoned.

[54] SILK SCREEN PRINTING MACHINE 10 Claims, 1 1 Drawing Figs.

[52] 11.8. C1. 101/124, 101/123,101/425 [51] Int. Cl 1141127/34 [50] Field ofSearch 101/116, 120,123,124, 425; 118/406; 15/256.51;29/122 [56] References Cited UNITED STATES PATENTS 1,317,713 10/1919 Linder 29/122 1,541,787 6/1925 Cadgene et a1. 101/120 2,551,601 5/11951 Hughson 101/425 2,732,591 1/1956 Whittum 29/122 X 3,046,880 7/1962 Black 101/123 3,318,237 5/1967 Ramsey 101/124 3,408,934 1 111968 Clausen 101/425 FOREIGN PATENTS 801,556 9/1958 Great Britain 101/116 Primary Examiner-Clyde l. Coughenour 3 Attorney-Dunlap, Laney, Hessin & Dougherty paper.

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PATENTED mm 5 I971 SHEET [1F 4 INVIZNTOR.

SILK SCREEN PRINTING MACHINE RELATED APPLICATIONS This application is a continuation-in-part of my US. application Ser. No. 697,232 filed .Ian. 11, 1968 now abandoned, entitled Silk Screen Printing Machine.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to silk screen printing apparatus, and more particularly, to such apparatus in which the screen is disposed in a generally horizontal position, and the printing medium is forced through the screen from the underside thereof by means of a continuously cleaned pressure roller rotated about an axis which extends parallel to the plane of the screen.

2. Brief Description of the Prior Art This invention constitutes an improvement in the silk screen printing machine described in my U.S. Pat. No. 3,318,237, which patent is incorporated by reference in this application. In the machine described in the cited patent, a stencil or silk screen is mounted on a suitable, horizontally reciprocating bed or platen which positions the screen below a rotating drum carrying, on its periphery, the paper which is to be printed. Ink is transferred from an ink reservoir to the underside of the silk screen by means of an inking roller and a cooperating inking blade. The inking blade disclosed in the patented apparatus is an elongated plate having a flat surface for receiving and supporting ink when an edge of the inking blade contacts the periphery of the inking roller. The blade, at this time, extends into juxtaposition to the silk screen (which is then being reciprocated in one direction in its printing stroke) and acts as a transfer surface across which ink is transferred into contact with the lower surface of the screen. During the nonprinting or retractive stroke of the screen-carrying platen, the inking blade is pivoted away from the screen and its ink supporting surface extends substantially horizontally.

The ink which is applied to the underside of the screen by the inking blade is distributed fairly evenly thereon with a spreader blade, and then a part of this ink is pressed through the openings in the screen onto the paper to be printed by a squeegee blade. This is accomplished as the squeegee blade moves upwardly to force the reciprocating silk screen against the paper carried on the periphery of the rotating drum.

Although my patented silk screen printing machine works well, I have conceived certain improvements to various parts of the device. These improvements reside generally in developments of a more efricient inking blade structure, and of a means for forcing ink through the screen onto the paper to be printed which is more versatile and better in several respects than the described squeegee blade method.

BRIEF AND GENERAL DESCRIPTION OF THE PRESENT INVENTION The present invention comprises a silk screen assembly of the general type disclosed in my US. Pat No. 3,318,237, but being improved in the apparatus provided for positioning ink on the screen, and for forcing it through the screen onto the paper to be printed. More specifically, I provide a concave or trowel-shaped inking blade which more efficiently accepts ink from an inking roller and transfers it to the underside of the screen. I also provide a pressure roller which rolls against the underside of the screen to force ink carried thereon through the openings in the screen and onto a rigidly held rotating semicylindrical paper surface. The pressure roller is of relatively large diameter in the center (as compared to the diameter of the ends of the roller) and has an external peripheral surface which tapers from the large-diameter central portion to the opposite end portions of the roller. In a preferred em- I For the purpose of preventing undesirable accumulations of ink on the pressure roller, a porous wiper is provided for periodically contacting the periphery of the roller over its length. A solvent is continuously fed to the porous wiper, and moves through the wiper to its wiping edge by capillary action. A partitioned tray is mounted beneath the pressure roller and the inking blade, and accommodates, in one chamber, ink to be transferred to the screen, and in another chamber residual ink and solvent which must be accommodated as a result of the periodic cleaning of the pressure roller.

The structural combination in which the described improvements appear can be described as comprising a cylindrical roll or drum adapted to carry on a portion of its outer periphery, a sheet of material, such as paper, which is to be printed; a bed or platen mounted below the drum for horizontal reciprocation, and for movement toward and away from the drum; a silk screen or stencil carried by the platen; and means for cyclically and periodically applying ink to the lower side of the screen or stencil, and then forcing it through the screen onto the material to be printed. This latter means includes the described improvements, Le, a concave, trowel-shaped inking blade; mechanism for cyclically and periodically moving the inking blade into juxtaposition to the underside of the screen; a pressure roller for periodically forcing ink through the screen onto the material to be printed; means for periodically moving the pressure roller into contact with the screen; a porous wiper element positioned to contact the periphery of the pressure roller when the pressure roller contacts the screen; and means for supplying solvent to the porous wiper element.

The described structure permits several important objects of the invention to be achieved, some of the more important of which are a. to make greater printing speeds possible;

b. to allow thinner inks to be printed, thereby reducing ink costs;

c. to allow finer detail to be printed with excellent fidelity;

d. to allow more nearly perfect register of the screen and the stock to be printed by eliminating or greatly reducing the frictional drag on the screen which is inherent in silk screen printing processes employing rubber or synthetic resin squeegee blades;

e. to increase the effective operating life of screens or stencils by reducing frictional drag thereon during the printing process; and

f. to more efficiently and uniformly transfer ink from an ink reservoir to the lower side of a stencil or silk screen moving in a horizontal plane.

There are other objects achieved by the invention, and other advantages which it possesses over previously known apparatus of this type. These objects and advantages, as well as the objects specifically mentioned, will be better appreciated when the following detailed description of the invention is read in conjunction with the accompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a silk screen printing machine constructed in accordance with the invention.

FIG. 2 is a schematic diagram illustrating a drive system used in the silk screen printing machine of FIG. I.

FIG. 3 is an enlarged perspective view illustrating the inking system utilized in the silk screen printing machine of FIG. 1.

FIG. 4 is an enlarged detail view of a portion of the silk screen printing machine illustrating the pressure roller and solvent feeding porous wiper in one position of operation.

FIG. 5 is a sectional view 'taken transversely through the pressure roller and solvent feeding porous wiper illustrating the pressure roller in a second, nonprinting position.

FIG. 6 is schematic diagram illustrating a portion of the apparatus used in the printing machine of FIG. 1 for applying anddistributing ink on the silk screen.

FIG. 7 is a schematic diagram illustrating the apparatus used in the printing machine of FIG. 1 for actuating the pressure roller.

FIG. 8 is a schematic diagram illustrating the drive system utilized in the printing machine of FIG. 1 for rotating the inking roller.

FIG. 9 is a schematic diagram illustrating the positions of the various components of the printing machine during the movement of the silk screen relatively toward the cylindrical printing drum and during the printing cycle.

FIG. 10 is a schematic view similar to FIG. 9, but illustrating the various parts of the printing machine during the movement of the silk screen relatively away from the cylindrical printing drum.

FIG. 1] is a perspective view of the pressure roller used in the machine of the invention.

Referring to the drawings, and initially to FIG. 1, a printing machine constructed in accordance with the present invention is there illustrated, and is designated generally by reference numeral 10. The printing machine 10 includes a base 12 having a bed or platen 14 mounted on the bed, and arranged to be reciprocated relative to the base 12 and to a printing drum 16. An angularly disposed platform 18 is mounted on the base 12 by means of vertical stanchions l9, and has one edge positioned adjacent the periphery of the drum 16. The platform 18 is provided so that sheets of paper stock 20 may be supported thereon and fed from this platform to the printing drum, as will be more fully described hereinafter.

The printing drum 16 is journaled in the base 12 and includes a pair of spaced grippers 21 for holding one end of the paper stock 19 in engagement with the drum. The grippers 21 are moved into and out of engagement with the paper stock 20 by suitable cams mounted on the base 12. Other grippers (see FIGS. 9 and 10) are disposed around the drum for gripping the other edge of the paper stock 20. The construction of the drum 16 and the grippers 21, and the manner in which paper stock is held on the drum during its rotation are well understood by those skilled in the art and constitute no part of the present invention.

The printing machine 10 further includes a plurality of belts 22 which extend around a pair of shafts 24 and 26 and these belts are positioned for moving the paper stock from the printing machine 10 to a paper conveyor 28 after the printing process has been completed. A silk screen frame 30 is mounted on the platen 14 by suitable fastening elements for reciprocating movement with the platen. A stencil or silk screen 32 is fastened to the frame 30. The stencil or silk screen 32 is of the usual construction, that is, it is fabricated from a porous material, such as silk, and has been prepared for use by closing a portion of the pores or interstices of the material to form the desired design thereon. An ink and solvent tray 34 is releasably secured to the base 12 by a plurality of brackets 35, and is located generally under the printing drum 16 and below the silk screen 32. The silk screen 32 is mounted so that it passes between the tray 34 and the drum 16, but above the platen 14.

The printing machine 10 is provided with a manual brake (not shown). A brake pedal 37 is shown in FIG. 1 and is connected through appropriate linkages 39, and others which are not shown, with the manual brake. Such manual brakes and their operating mechanism are conventional structures in silk screen mechanisms and are well understood by those skilled in the art.

As shown schematically in FIG. 2, the printing machine 10 is driven by a motor 40 that is mounted on the base 12. The motor 40 includes an output shaft 42 having a flywheel 44 and a pair of gears 46 and 48 mounted thereon. The gear 46 meshes with a mating gear 50 that is connected for rotation with the printing drum 16. The gear 48 meshes with a mating gear 52 that is mounted on a shaft 54 journaled in the base 12 by suitable bearings. The shaft 54 carries a gear 56 that meshes with a gear 58 on a drive shaft 60. The drive shaft 60 carries a second gear 62 that is alternately engageable with rack gears 64 and 66, each of which is mounted on the platen 14. The gear 52 carried on the shaft 54 is also in mesh with a gear 68 mounted on a cam shaft 70 that is journaled in the base 12 by suitable bearings. The camshaft 70 also has four cams 74, 76, 78 and 80 mounted thereon and positioned exteriorly of the base 12.

The gear arrangement described for driving the printing machine 10 is constructed so that the drum 16 makes one revolution during the movement of the platen 14 relatively toward the drum, and makes one additional revolution during the movement of the bed relatively away from the drum; that is, the drum makes two revolutions for each reciprocating movement of the platen. The camshaft 70 makes one revolution for each two revolutions of the drum 16; that is, the cam shaft 70 rotates once for each complete reciprocating movement of the platen 14.

FIG. 3 is an enlarged view showing the printing machine inking system in relation to the printing drum 16. The base 12 has been omitted from this view in order to present the several structural elements more clearly. As shown in FIG. 3, three shafts 82, 84 and 86 are disposed and spaced in generally parallel relationship relatively in front of the drum 16. Each of the shafts 82, 84 and 86 is journaled at each end in brackets 88 that are attached to the base 12 as shown in FIG. 1. A pair of spaced lever arms 90 are rigidly attached by one of their ends to the shaft 82. The other end of each lever arm 90 is con nected to one end of a linkage member 92. The other ends of the linkage members 92 are removably and pivotally connected with a four bar linkage 94 that is pivotally mounted on a bracket 96 attached to the tray 34. The four bar linkage 94 has an elongated spreader blade 98 mounted thereon and extending transversely across the printing machine 10 below the silk screen or stencil 32. The linkage 94 also carries an elongated inking blade 100 which, in accordance with the present invention, is concave or trowel-shaped in cross-sectional configuration and extends across the printing machine 10 at a location below the stencil 32. As depicted, the inking blade 100 includes a baseplate 100a, and a pair of sideplates 100 b which extend outwardly and upwardly from the baseplate.

The longitudinal edge of one of the sideplates 100b of the inking blade 100 is disposed adjacent an elongated inking roller 104 that is journaled in the frame 12. The longitudinal edge of the second sideplate 1001; of the inking blade 100 extends parallel to the stencil 32. The blade 100 is moved during operation of the device to an ink receiving position in which one of its longitudinal edges is in juxtaposition to the inking roller 104 and its other edge is in juxtaposition to the lower surface of the stencil 32. This positioning of the inking blade 100 occurs during the printing cycle and functions to transfer ink from the inking roller 104 to the lower side of the stencil.

As illustrated in the exploded portion of FIG. 3, the ends of the shaft which carries the inking roller 104 are each removably journaled in the base 12 by an assembly comprising a bracket 106 that is rigidly mounted on the base. The bracket 106 has a recess 108 formed in one of its sides to receive a bearing 1 10. The bearing 110 is provided with an aperture 109 for receiving the end of the shaftwhich carries the inking roller 104. The bracket 106 is also provided with a hole 112 arranged to receive a pin 114 which extends from the lower side of a clamping member 116. The inking roller 104 is mounted in the bracket 106 by placing the ends of its shaft through the apertures 109 in the respective bearings 110. These bearings 110 are then positioned in the recess 108 in the respective brackets 106. The clamping member 116 is then connected with the bracket 106 by inserting the pin 114 into the hole 112. A dog 118 which is pivotally mounted in the base 12 is then rotated downwardly into a notch 120 provided in the upper surface of the clamping member 116, thereby mounting the clamping member 116 in assembled relation with the bracket 106. A threaded thumbscrew 122 that extends through the clamping bracket 116 is then tightened so that an end thereof (not shown) engages the bearing 110 to secure the bearing in the bracket. The described assembly permits the inking roller 104 to be quickly and easily removed from the base 12.

As illustrated in FIG. 3, a spur gear 124 is mounted on the shaft carrying the inking roller 104 and meshes with a spur gear 126 mounted on a stub shaft 128. The stub shaft 128 is joumaled in the base 12 and carries a chain sprocket 130. The spur gears 124 and 126 and the chain sprocket 130 are part of the mechanism for rotating the inking roller 104, which mechanism will be described more fully hereinafter.

The shaft 84 has a pair of levers 132 connected at one end thereof to the shaft and pivotally connected at their other ends to one end of a respective one of a pair of linkage members 134. The other end of each of the linkage members 134 is pivotally and releasably connected to a lever 136. The lever 136 is pivotally mounted in the base by a suitable pin (not illustrated) which is inserted through an opening 138 provided adjacent the opposite end of the lever 136 from the end connected to the linkage member 134. The lever 136 of the present invention includes a vertically extending plate 136a which is secured at its upper edge to a horizontally extending plate 136b. The horizontally extending plate l36b of the lever 136 has a substantially rectangular or flat-sided slot 140 formed therein (see FIGS. 3 and 4) to accommodate or receive the rectangularly cross-sectional end portion of an elongated shaft 142. The end portions of the shaft 142 are machined to fit in the rectangular slots 140 by machining flats on the opposite sides thereof so that when the end portions of the shaft are positioned in the slots 140 in the levers 136 located on opposite sides of the machine, the shaft 142 is prevented from rotating. Joumaled on the elongated shaft 142 for rotation thereabout is a pressure roller 144. The pressure roller 144 extends substantially parallel to the rotational axis of the printing drum 16, and is located immediately below the drum and on the opposite side of the stencil or silk screen 32 from the drum. It will be noted from the description of the manner in which the pressure roller 144 is mounted in the levers 136 that the pressure roller can be easily removed from these levers for purposes of maintenance or cleaning by merely removing the end portions of the shaft 142 from the rectangular slots 140.

The pressure roller is preferably constructed from a material, such as a resilient natural or synthetic rubber and, as will be hereinafter described, is movable into and out of engagement with the stencil 32 for the purpose of transferring ink through the stencil to the paper stock carried on the drum 16. The geometric configuration of the pressure roller will also be hereinafter described in greater detail.

For the purpose of cleaning residual ink from the pressure roller 144 during the operation of the machine, a solvent feeding assembly is provided, and is designated generally by reference numeral 146 in the drawings. The solvent feeding assembly 146 includes an elongated wick socket 148 which receives and contains the base of a porous wick member or wiper 150. The porous wiper 150 extends outwardly from the wick socket 148 to a position in which it contacts the periphery of the pressure roller 144 when the pressure roller is elevated to the printing position depicted in FIG. 4. The solvent feeding assembly 146 further includes a plurality of solvent feed pipes 152 which extend from a solvent manifold line 154 to the wick socket 148 and communicate with the interior of the wick socket. A solvent is continuously fed under very low pressure to the wick socket 148 from the solvent manifold line 154 via the solvent feed pipes 152. The arrangements of the pressure roller 144 and the solvent feeding assembly 146 are best illustrated in FIGS. 4 and 5.

A pair of levers 158 are connected to the shaft 86, and to one end of a respective one of a pair of linkage members 160. The other ends of each of the linkage members 160 are each pivotally and removably connected to a wiper blade mounting plate 161. The wiper blade mounting plates 161 have secured thereto a wiper blade 162. Each of the wiper blade mounting plates 161 is pivotally connected to the bracket 96 hereinbefore described. The wiper blade 162 extends transversely across the machine 10 below the stencil 32. The wiper blade 162 is movable into and out of engagement with the underside of the stencil 32 during operation of the machine as will be more fully hereinafter described.

In the present invention, a suitable system may be incorporated in the machine for spraying solvent on the stencil 32 after the printing cycle has occurred. A suitable system for spraying solvent on the upper side of the stencil to remove residual ink therefrom is illustrated and described in my US. Pat. No. 3,318,237, and is also depicted in part in this application, where it is designated by reference numeral 161. Since this system does not, however, constitute a portion of the improvements to the machine constituting the present invention, no further reference will be made to this system in the present application.

The schematic diagram of FIG. 6 illustrates more completely the structure for moving the inking blade and the spreader blade 98 into a position where ink will be deposited on the underside of the stencil 32. As here illustrated, a third lever 163 is mounted on the shaft 82 and is pivotally connected with one end of a linkage member 164. The other end of the linkage member 164 is pivotally connected with a cam follower 165. The cam follower 165 is pivotally mounted in a bracket 166 which is connected to the base 12. The cam follower 164 is in engagement with the cam 76 carried by the camshaft 70 (see FIG. 2). Attached to the cam follower 165 is an upwardly extending rod 168 that extends through a bracket 170 mounted on the base 12 as shown in FIG. 1. A compression spring 172 is disposed on the rod 168 and has one end in engagement with the rod and the other end in engagement with the bracket 170 to resiliently bias the cam follower 165 into engagement with the cam 76. A lever 174 is pivotally mounted on the base 12 and has one end connected with the cam follower 165 by a member 176. The other end of the lever 174 is connected by a tension spring 178 with the base 12. A foot-operated pedal 180 extends through an operators platform 182 mounted on the base 12. The lower end of the pedal 180 is pivotally connected with the lever 174 between the pivotal mounting part of the lever and the end thereof which is attached to the tension spring 178. As can be appreciated from viewing FIG. 6, depressing the pedal 180 pivots the lever 174 relatively against the tension spring 178, forcing the cam follower 165 out of engagement with the surface of the cam 76. As long as the pedal 180 is held depressed, the spreader blade 98 and inking blade 100 are held in a spaced position relative to the stencil 32 so that ink cannot be deposited on the stencil. These positions of spreader blade and inking blade 100 are best illustrated in FIG. 10.

With the pedal 180 released, the cam follower 165 engages and follows the surface of the cam 76. It can be seen that the cam follower 165 moves relatively upwardly and downwardly depending on the particular portion of the cam 76 with which it is in engagement. When the cam follower 165 is in the position shown in FIG. 6, that is, in the position relatively close to the cam shaft 70, the linkage member 164 transmits a force to the lever 163 that rotates the shaft 82 and the lever arms 90, pulling the linkage members 92 relatively toward the shaft 82, and through the four bar linkage 94, raising the spreader blade 98 into juxtaposition with the underside of the stencil 32. SImultaneously, the inking blade 100 is moved upwardly into engagement wit the underside of the stencil 32. With the blades 98 and 100 in the position described, ink is transmitted from the inking roller 104 to the inking blade 100, from which the ink is deposited on the underside of the stencil. The spreader blade 98 distributes the ink over the underside of the stencil 32 and removes excess ink therefrom.

The concave or trowel-shaped inking blade 100 of the present invention permits ink to flow more smoothly and efficiently from the inking roller 104 to the under surface of the stencil 32. Moreover, when the inking blade 100 is lowered as hereinafter described and as depicted in FIG. 10, a small amount of ink is still received from the periphery of the inking roller 104, and the substantially horizontal position of the inking blade 100 permits its concavity to retain a reserve supply of ink so that when it is again elevated during the printing stroke of the machine, ink is more immediately applied to the underside of the stencil than is the case when a flat substantially monoplanar inking blade is employed.

When the cam 76 is rotated to a position wherein the cam follower 165 is raised upwardly, the shaft 82 is pivoted in the opposite direction to move the four bar linkage 92 relatively away from the shaft 82, moving the inking blade 100 and spreader blade 98 relatively away from the stencil 32 to the position shown in FIG. 10.

FIG. 7 illustrates schematically, but more completely, the structure for moving the pressure roller 144 into engagement with the underside of the stencil 32. As shown therein, a cam follower 184 is pivotally connected to the bracket 166, and is in engagement with the irregular surface of the cam 78 mounted on the cam shaft 70. A linkage member 186 extends from the cam follower 184 and is pivotally connected with a third lever 188 that is mounted on the shaft 84. An upwardly extending rod 190 connected at its lower end with the cam follower 184 extends through the bracket 170. A compression spring 192 that encircles the rod 190 has one end in engagement with the rod 190 and the other end in engagement with the bracket 170 whereby the cam follower 184 is resiliently biased into engagement with the surface of the cam 78. A connecting member 194 pivotally connects the end of the cam follower 184 with the lever 174. The lever 174 and various parts associated therewith function as described in connection with FIG. 6 to move the cam follower 184 out of engagement with the cam 78, thereby holding the pressure roller 144 out of engagement with the underside of the stencil 32 when the pedal 180 is depressed.

As illustrated in FIG. 7, the cam follower 184 is in engagement with the lower portion of the cam 78 so that the cam follower is moved relatively downwardly, pulling the lever 188 downwardly through the linkage member 186, pivoting the shaft 84 in a direction to lift the lever 132. This movement of the lever 132 lifts the lever 136 and the pressure roller 144 which it supports is moved into contact with the underside of the stencil 32. The stencil 32, due to the engagement of the pressure roller 144 therewith, is deformed upwardly into engagement with the paper stock carried by the drum 16. The pressure roller 144 forces the ink on the underside of the stencil 32 through the interstices or openings therein to deposit the ink on the surface of the paper stock in accordance with the pattern on the stencil.

When the camshaft 70 and the attached cam 78 rotate until the cam follower 184 is riding on the relatively high portion of the cam, the linkage member 186 moves upwardly, pivoting the shaft 84 in the opposite direction, pivoting the lever 136 relatively downwardly and moving the pressure roller which it supports out of engagement with the stencil and out of engagement with the porous wiper 150.

The mechanism by which the wiper blade 162 is periodically moved into and out of engagement with the underside of the stencil 32 is of conventional construction and is described in detail in my U.S. Pat. No. 3,318,236. Since neither this mechanism nor the wiper blade, nor the manner in which the mechanism and wiper blade function are per se novel, this mechanism shall not be further dealt with herein except to briefly mention the role played by the wiper blade 162 in the overall operation of the machine. This role will be discussed briefly hereinafter.

FIG. 8 illustrates schematically the drive system which is utilized for rotating the inking roller 104. Although the inking roller 104 may be driven by the motor 40 through an appropriate drive mechanism, it is preferred that a separate drive for this roller be provided in the manner illustrated in FIG. 8. It has been found that a different rotational speed of the inking roller 104 for different viscosities of printing media provides the most efficient movement of the ink from the roller 104 to the inking blade 100. Therefore, the chain sprocket 130 previously described, is connected by an endless chain 196 with a sprocket 198 mounted on an output shaft 200 of a variable speed device 202. The variable-speed device 202 is arranged to be driven by an electric motor 204 mounted on the base 12 as illustrated in FIG. 1. The variable speed device 202 is conveniently provided with a handle 206 for adjusting the speed of the output shaft 200 and the sprocket 198. A variable-speed motor may be utilized if desired in place of the motor 204 and the variable-speed device 202.

The pressure roller which is preferably used in the machine of the invention is illustrated in greater detail in FIG. 11 of the drawings. This preferred form of pressure roller 144 has its largest outside diameter at the center of the roller and tapers from this to a relatively smaller diameter adjacent the ends of the roller. The peripheral surface of the pressure roller may thus be curved parabolically, as shown in FIG. 11, or the roller may be formed as two truncated cones having their bases abutting at the center of the roller.

I have determined that superior printing results are attained when the pressure roller is shaped in the manner described. In the context in which this roller is employed, space and weight limitations dictate that the pressure roller be relatively long in proportion to its diameter-of the order of about 40 inches long to about 2 2 inches outside diameter. The bore through the roller 144 which accommodates the shaft 142 must be sized to receive a shaft not exceeding about 1% inch in diameter, and this shaft, as measured between its points of support at opposite ends thereof, will typically be about inches in length. At the time the pressure roller 144 is moved intocontact with the screen 32 to force ink therethrough, it is subjected to a high pressure forcing it against the screen backed up by the drum 16. Because of this force and the high lengthto-diameter ratio of the shaft 142, the shaft tends to bow or bend away from the screen at the central portion of the pressure roller which is spaced farthest, of course, from the points of support at the ends of the shaft 142.

It has been my observation that due to this bowing tendency, the pressure exerted on the central portion of the screen by the pressure roller 144 is less than that which is brought to bear near the ends of the roller in instances where the pressure roller is of uniform diameter over its length. This in turn results in nonuniform printing due to uneven distribution of ink through the screen. By making the pressure roller 144 of the configuration depicted in FIG. 11 in which the center is of slightly larger diameter than the ends, I have been able to overcome this problem and achieve unifonn ink distribution through the screen. A typical enlargement of the i type described entails providing a central diameter which is from about one-fourth inch to three-fourths inch larger than the diameter of the roller at each of its ends where the length of the roller is from about 35 inches to 50 inches. This characterization, however, is certainly not limiting with respect to the use of the described principle, since the type of material of which the roller is made, the strength of the shaft material and other factors will, in various cases, dictate departures from the typical dimensions stated.

OPERATION The printing machine 10 is placed in operation by starting the motor 36 and releasing the brake pedal 37. As described in the discussion of FIG. 2, the motor 40 reciprocates the platen 14 and causes the rotation of the camshaft and the printing drum 16. Also, the motor 204 is started to rotate the inking roller 104.

The paper stock 20 from the platfonn 18 is fed to the drum 16 where it is engaged by the grippers 21 so that the paper stock is disposed around the periphery of the drum in the manner illustrated in FIGS. 9 and 10. As the drum 16 rotates, carrying the paper stock 20 therewith, the platen 14 moves relatively toward the drum 16 due to the engagement of the gear 62 with the rack gear 66 (see FIG. 9). During the movement of the platen 14 toward the drum 16, the wiper blade 162 is held out of engagement with the underside of the stencil 32 in the manner shown in FIG. 9 by an operating mechanism which cooperates with the cam 74 carried on the cam shaft 70.

This operating mechanism is described in my US. Pat. No.

3,318,237. The spreader blade 98 and inking blade 100 are moved upwardly into juxtaposition to the underside of the stencil 32 since the cam follower 165 (see FIG. 6) connected therewith is in engagement with the lower portion of the cam 76.

Ink or other printing medium contained in the ink reservoir of the tray 34 is picked up by the inking roller 104 and carried thereby to the upper or dished surface of the concave inking blade 100. The ink flows acros the upper surface of the inking blade 100 into contact with the underside of the stencil 32. As the platen 14 and stencil 32 move relatively toward the drum 16, the ink of the underside of the stencil 32 is distributed evenly on the stencil by the spreader blade 98 that is in contact with the underside of the stencil. As the inked portion of the stencil 32 approaches the pressure roller 144, this roller is moved upwardly into engagement with the underside of the stencil, since the cam follower 184 (see FIG. 7) connected therewith is on the lower surface of the cam 78. Simultaneously with the contact of the pressure roller 144 with the lower side of the stencil 32, the periphery of the pressure roller is brought into wiping engagement with the porous wiper 150.

The engagement of the pressure roller 144 with the stencil 32 defonns the stencil relatively upwardly into contact with the paper stock 20 carried by the drum 16. It will also be noted that as the platen 14 and the stencil 32 continue to reciprocate between the pressure roller 144 and the drum 16, the pressure roller is free to turn about its axis and, due to its frictional engagement with the stencil 32, will rotate and exert little frictional drag on the stencil. This constitutes an important feature of the present invention and permits many of the heretofore enumerated objects to be achieved. Due to the pressure of the pressure roller 144 on the lower side of the stencil 32 while the stencil contacts the paper stock 20 carried by the drum 16, the ink on the stencil is forced through the small openings therein, thus leaving an imprint on the paper stock 20. The continuing traverse of the stencil 32 results in the particular pattern or print desired being transferred to the paper stock.

During a portion of the operating cycle when the pressure roller 144 engages the lower side of the stencil 32 to force ink through the stencil onto the paper stock 20, the periphery of the pressure roller is in engagement with the porous wiper 150. A suitable liquid solvent is continuously introduced to the wick socket 148 from the solvent manifold line 154 and solvent feed pipes 152. The liquid solvent enters the pores of the porous wiper or wick 150 and moves by capillary action through this porous wiper to the edge thereof which is in contact with the pressure roller 144. A continual cleaning of'the surface of the pressure roller 144 thus occurs so that a buildup of residual hardened or crusted ink does not occur thereon, and the surface is clean and smooth at all times. Solvent applied to the outer periphery of the pressure roller 144 may drip from this roller into the solvent compartment of the tray 34 and can be removed from this tray through a solvent waste line 210 and reclaimed for reuse in the system.

When the stencil 32 has completed its reciprocating movement relative to the printing drum 16, the gear 62 moves out of engagement with the rack gear 66 and into engagement with the rack gear 64 (see FIG. thereby reversing the direction of movement of the platen 14 and the attached stencil. As this occurs, the pressure roller 144 is moved out of engagement with the stencil 32, since the cam follower 184 connectedtherewith is in engagement with the raised surface of the cam 78 (see FIG. 7). Simultaneously, or shortly thereafter, the spreader blade 98 and the concave inking blade 100 are moved downwardly from the stencil 32 as the cam follower 165 connected therewith (see FIG. 6) engages the raised surface of the cam 76. It will be noted in referring to FIG. 10 that the movement which is undergone by the inking blade 100 is such that this blade tends to pivot to a substantially horizontal position in which its edge nearest the inking roller 104 is still positioned to receive some ink from the inking roller. The inking blade thus tends to have its concavity filled or at least partially filled, with ink during the time that it is in its inoperative position from the standpoint of its primary function of transferring ink from the inking roller 104 to the underside of the stencil 32. This arrangement thus permits ink to be readily available for transfer to the underside of the stencil 32 at such time as another printing cycle is commenced and the inking blade 100 is moved upwardly to its ink transfer position shown in FIG. 9.

As is well understood in the art, during the retraction or nonprinting stroke of the platen 14, in which the stencil is moved to the left as viewed in FIG. 10, the'wiper blade 162 is moved upwardly into engagement with the underside of the stencil 32 by a mechanism responsive to the rotation of the cam 74 carried on the camshaft 70. The wiper blade 162 functions at this time to remove any residual ink which remains on the bottom of the stencil.

As the first revolution of the printing drum 16 is completed, the grippers 21 open to release the paper stock 20 so that the printed stock is discharged from the periphery of the drum onto the belts 22, thus completing the printing cycle. It should be pointed out that the stencil 32 moves out of engagement with the printing drum 16 when the pressure roller 144 moves to its lower position as illustrated in FIG. 10. The only member in wiping contact with the stencil 32 during the movement of the stencil relatively away from the printing drum 16 is the wiper blade 162.

The tray 34 and the various blades and the pressure roller 144 which have been hereinbefore described are mounted in the machine 10 in a relatively stationary position, that is, they do not reciprocate. Therefore, the only reciprocating portion of the machine 10 is the platen l4 and the stencil or silk screen 32 which it carries. This permits very rapid operation of .the machine 10 without the danger of splashing or spilling ink from the tray 34. Also, the machine remains relatively clean during its operation, due to the continuous cleaning of the pressure roller 144 when it is in contact with the inked stencil, and the cleaning actions of the spreader blade 98 and the wiper blade 162.

If the machine is to be shut down for a period of time, the pedal 180 is depressed, moving the spreader blade 98, inking blade 100 and pressure roller 144 out of engagement with the stencil 32 so that no ink can be deposited thereon or transferred therethrough.

If a different type or color of printing medium is to be used in the machine 10, the linkage members 92, 134 and are removed from their respective blades. The ends of the inking roller 104 are released from their mounting in the base 12 by removing the clamping members 116 as previously described. The tray 34, after the brackets 35 have been removed, is pulled from the machine 10 along with the blades 98, 100 and 162 for cleaning or replacement. Also, the pressure roller 144 may be quickly and easily removed from the machine merely by lifting the ends of the shaft 142 from the rectangular recesses or notches 140. Thus, it can be seen that the machine 10 is arranged so that the portions thereof that are in contact with the ink or printing medium can be quickly and easily removed when multiple color prints are to be made, or when the machine 10 is to be cleaned.

The embodiments of my improvements in silk screen printing machines which have been herein described are intended as examples sufficient to permit interested and authorized parties to practice my invention without difficulty. I have not undertaken to set forth any and all forms which such improvements may take within the widest scope of the inventive principles herein disclosed. It will be appreciated that some changes of structure and form can be efi'ected without departing from such principles. it is therefore my intention that all changes and modifications which continue to rely upon the basic principles underlying the present invention and the preferred embodiment herein described shall be circumscribed by the spirit and scope of the invention, except as such spirit and scope may be necessarily limited by the appended claims or reasonable equivalents thereof.

What is claimed is:

1. In a printing machine of the type having a printing drum for carrying, on the periphery thereof, paper stock to be printed, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation thereof in a plane extending substantially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising:

a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum, said pressure roller having a relatively large diameter in the central portion thereof as contrasted with relatively smaller diameters at opposite ends thereof, and having a peripheral surface tapering in opposite directions from the central portion of the pressure roller to the opposite ends thereof;

means for synchronously moving said pressure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil to a second position in which said pressure roller is forced against the lower side of said stencil to contact the stencil with an even pressure over the length of the roller and thereby press said stencil against the stock to be printed carried on the periphery of said drum;

a porous wiper positioned for contacting the periphery of said pressure roller over its length at the time when said pressure roller is in said second position and bears against the lower side of said stencil, and to be out of contact with said pressure roller when said pressure roller is in said first position; and

means for continuously feeding a cleaning solvent to said porous wiper.

2. In a printing machine of the type having a printing drum for carrying paper stock to be printed on the periphery thereof, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation thereof in a plane extending substantially tangentially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising:

a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum;

means for synchronously moving said pressure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil to a second position in which said pressure roller bears against the lower side of the stencil to press said stencil against a stock to be printed carried on the periphery of said drum;

means for periodically cleaning ink from the periphery of said pressure roller by applying a cleaning solvent thereto;

a partitioned tray including an ink chamber and a cleaning solvent chamber therein, said ink chamber forming said ink reservoir and said solvent chamber being positioned beneath said pressure roller for receiving residual cleaning solvent from said pressure roller; and

means for continuously removing cleaning solvent from said solvent chamber.

3. The improvement in a printing machine defined in claim 2 wherein said ink cleaning means comprises:

a porous wiper positioned for contacting the periphery of said pressure roller over its length at the time when said pressure roller is in said second position and bears against the lower side of said stencil, and to be out of contact with said pressure roller when said pressure roller is in said first position; and

means for continuously feeding cleaning solvent to said porous wiper.

4. The improvement in a printing machine defined in claim 2 and further characterized to include:

an elongated inking roller positioned in said ink chamber for receiving ink on the periphery of said inking roller;

a concave inking blade having a first longitudinal edge adjacent the peripheral surface of said inking roller and a second longitudinal edge spaced from said first longitudinal edge and extending parallel to the plane of said stencil; and

means for moving said inking blade in synchronism with said platen from a first position, in which said longitudinal edge of said inking blade is in juxtaposition to the peripheral surface of said inking roller and said second longitudinal edge is in juxtaposition to said stencil, to a second position in which said second longitudinal edge is spaced downwardly from said stencil.

5. The improvement in a printing machine defined in claim 4 wherein said inking blade includes an elongated baseplate and a pair of side plates extending upwardly from said baseplate and each having a free upper edge, said free upper edges constituting said first and second longitudinal edges, respectively.

6. in a printing machine of the type having a printing drum for carrying paper stock to be printed on the periphery thereof, and mounted for reciprocation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation therewith in a plane extending substantially tangentially with respect to said drum and an ink reservoir mounted beneath said stencil, the improvement comprising:

a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum;

a pair of horizontally spaced levers each having a flat-sided,

open-topped slot therein;

a shaft extending through, and rotatably journaling, said pressure roller, said shaft having flats formed on the sides thereof at the opposite ends thereof, with the portions of said shaft carrying said flats positioned in said slots and cooperating with said slots to prevent rotation of said shaft about its axis; and

means connected to said levers for periodically pivoting said levers through an arc to move said pressure roller between a first position out of contact with said stencil, and a second position in which said pressure roller bears against the lower side of said stencil to press said stencil against a stock to be printed carried on the periphery of said drum.

7. In a printing machine of the type having a printing drum for carrying paper to be printed on the periphery thereof, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation therewith in a plane extending substantially tangentially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising:

an inking roller mounted in said ink reservoir in a position to receive ink therefrom on the periphery of said roller;

a concave trowel-shaped inking blade positioned above said ink reservoir and adjacent said inking roller, said inking blade having its concavity opening upwardly so that the blade can retain ink, and having a first longitudinal edge positioned adjacent the periphery of said roller, and a second longitudinal edge extending parallel to said first longitudinal edge, and to the plane of said stencil; and

means for moving said inking blade from a first position in which said second longitudinal edge is in juxtaposition to the lower surface of said stencil to a second position in which said second longitudinal edge is spaced downwardly from said stencil.

8. The improvement in a printing machine as defined in claim 7 and further characterized to include a pressure roller spaced from said inking roller and inking blade in the direction of said printing drum, and positioned on the opposite side of said stencil from said drum for pressing the stencil in the direction of the drum.

9. The improvement in a printing machine as defined in claim 8 and further characterized to include means for continuously cleaning said pressure roller while it is in contact with said stencil.

10. A printing machine comprising:

a cylindrical printing drum for detachably carrying paper stock to be printed on the periphery thereof, and mounted for rotation about a horizontal axis;

a reciprocating platen mounted for reciprocation in a substantially horizontal direction to a position beneath said drum;

a stencil carried by said platen for reciprocation thereof in a plane extending substantially tangentially with respect to said drum;

means for periodically applying ink to the underside of said stencil in synchronism with the reciprocation of said platen and the stencil carried thereby;

a pressure roller having an enlarged diameter central portion and having a peripheral surface tapering in opposite directions from the central portion thereof to the opposite ends thereof, said pressure roller being movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said printing drum;

means for synchronously moving said presure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil, to a second position in which said pressure roller bears with substantially uniform pressure over its length against the lower side of said stencil to press said stencil against the stock to be printed carried on the periphery of said drum, said means for synchronously moving said pressure roller including horizontally spaced levers detachably supporting said pressure roller at opposite ends of the pressure roller; and

means connected to said levers for periodically pivoting said levers through an arc to move said pressure roller between said first and second positions; and

a wiper periodically contacting the periphery of said pressure roller over its length at a time when said pressure roller is in said second position, and being spaced from the periphery of said pressure roller when said pressure roller is in said first position. 

1. In a printing machine of the type having a printing drum for carrying, on the periphery thereof, paper stock to be printed, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation thereof in a plane extendIng substantially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising: a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum, said pressure roller having a relatively large diameter in the central portion thereof as contrasted with relatively smaller diameters at opposite ends thereof, and having a peripheral surface tapering in opposite directions from the central portion of the pressure roller to the opposite ends thereof; means for synchronously moving said pressure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil to a second position in which said pressure roller is forced against the lower side of said stencil to contact the stencil with an even pressure over the length of the roller and thereby press said stencil against the stock to be printed carried on the periphery of said drum; a porous wiper positioned for contacting the periphery of said pressure roller over its length at the time when said pressure roller is in said second position and bears against the lower side of said stencil, and to be out of contact with said pressure roller when said pressure roller is in said first position; and means for continuously feeding a cleaning solvent to said porous wiper.
 2. In a printing machine of the type having a printing drum for carrying paper stock to be printed on the periphery thereof, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation thereof in a plane extending substantially tangentially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising: a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum; means for synchronously moving said pressure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil to a second position in which said pressure roller bears against the lower side of the stencil to press said stencil against a stock to be printed carried on the periphery of said drum; means for periodically cleaning ink from the periphery of said pressure roller by applying a cleaning solvent thereto; a partitioned tray including an ink chamber and a cleaning solvent chamber therein, said ink chamber forming said ink reservoir and said solvent chamber being positioned beneath said pressure roller for receiving residual cleaning solvent from said pressure roller; and means for continuously removing cleaning solvent from said solvent chamber.
 3. The improvement in a printing machine defined in claim 2 wherein said ink cleaning means comprises: a porous wiper positioned for contacting the periphery of said pressure roller over its length at the time when said pressure roller is in said second position and bears against the lower side of said stencil, and to be out of contact with said pressure roller when said pressure roller is in said first position; and means for continuously feeding cleaning solvent to said porous wiper.
 4. The improvement in a printing machine defined in claim 2 and further characterized to include: an elongated inking roller positioned in said ink chamber for receiving ink on the periphery of said inking roller; a concave inking blade having a first longitudinal edge adjacent the peripheral surface of said inking roller and a second longitudinal edge spaced from said first longitudinal edge and extending parallel to the plane of said stencil; and means for moving said inkIng blade in synchronism with said platen from a first position, in which said longitudinal edge of said inking blade is in juxtaposition to the peripheral surface of said inking roller and said second longitudinal edge is in juxtaposition to said stencil, to a second position in which said second longitudinal edge is spaced downwardly from said stencil.
 5. The improvement in a printing machine defined in claim 4 wherein said inking blade includes an elongated baseplate and a pair of side plates extending upwardly from said baseplate and each having a free upper edge, said free upper edges constituting said first and second longitudinal edges, respectively.
 6. In a printing machine of the type having a printing drum for carrying paper stock to be printed on the periphery thereof, and mounted for reciprocation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation therewith in a plane extending substantially tangentially with respect to said drum and an ink reservoir mounted beneath said stencil, the improvement comprising: a pressure roller movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said drum; a pair of horizontally spaced levers each having a flat-sided, open-topped slot therein; a shaft extending through, and rotatably journaling, said pressure roller, said shaft having flats formed on the sides thereof at the opposite ends thereof, with the portions of said shaft carrying said flats positioned in said slots and cooperating with said slots to prevent rotation of said shaft about its axis; and means connected to said levers for periodically pivoting said levers through an arc to move said pressure roller between a first position out of contact with said stencil, and a second position in which said pressure roller bears against the lower side of said stencil to press said stencil against a stock to be printed carried on the periphery of said drum.
 7. In a printing machine of the type having a printing drum for carrying paper to be printed on the periphery thereof, and mounted for rotation about a horizontal axis, a reciprocating platen mounted beneath said drum for reciprocation in a substantially horizontal direction, a stencil carried by said platen for reciprocation therewith in a plane extending substantially tangentially with respect to said drum, and an ink reservoir mounted beneath said stencil, the improvement comprising: an inking roller mounted in said ink reservoir in a position to receive ink therefrom on the periphery of said roller; a concave trowel-shaped inking blade positioned above said ink reservoir and adjacent said inking roller, said inking blade having its concavity opening upwardly so that the blade can retain ink, and having a first longitudinal edge positioned adjacent the periphery of said roller, and a second longitudinal edge extending parallel to said first longitudinal edge, and to the plane of said stencil; and means for moving said inking blade from a first position in which said second longitudinal edge is in juxtaposition to the lower surface of said stencil to a second position in which said second longitudinal edge is spaced downwardly from said stencil.
 8. The improvement in a printing machine as defined in claim 7 and further characterized to include a pressure roller spaced from said inking roller and inking blade in the direction of said printing drum, and positioned on the opposite side of said stencil from said drum for pressing the stencil in the direction of the drum.
 9. The improvement in a printing machine as defined in claim 8 and further characterized to include means for continuously cleaning said pressure roller while it is in contact with said stencil.
 10. A printing machine comprising: a cylindrical printing drum for Detachably carrying paper stock to be printed on the periphery thereof, and mounted for rotation about a horizontal axis; a reciprocating platen mounted for reciprocation in a substantially horizontal direction to a position beneath said drum; a stencil carried by said platen for reciprocation thereof in a plane extending substantially tangentially with respect to said drum; means for periodically applying ink to the underside of said stencil in synchronism with the reciprocation of said platen and the stencil carried thereby; a pressure roller having an enlarged diameter central portion and having a peripheral surface tapering in opposite directions from the central portion thereof to the opposite ends thereof, said pressure roller being movably supported on the opposite side of said stencil from said printing drum and rotatable about an axis extending parallel to the axis of rotation of said printing drum; means for synchronously moving said pressure roller in synchronism with the reciprocating movement of said platen to move said pressure roller from a first position out of contact with said stencil, to a second position in which said pressure roller bears with substantially uniform pressure over its length against the lower side of said stencil to press said stencil against the stock to be printed carried on the periphery of said drum, said means for synchronously moving said pressure roller including horizontally spaced levers detachably supporting said pressure roller at opposite ends of the pressure roller; and means connected to said levers for periodically pivoting said levers through an arc to move said pressure roller between said first and second positions; and a wiper periodically contacting the periphery of said pressure roller over its length at a time when said pressure roller is in said second position, and being spaced from the periphery of said pressure roller when said pressure roller is in said first position. 